The present invention relates to an image forming apparatus, such as a digital copying machine or a laser printer, which employs a beam scanning apparatus. The beam scanning apparatus forms one electrostatic latent image on a photosensitive drum by exposure by scanning a plurality of optical beams over the photosensitive drum at one time.
In recent years, digital copying machines of various types have been developed, which execute image formation by the scanning of optical beams and by use of an electrophotography process.
A digital copying machine that has been developed very recently is designed to speed up the image formation by adopting a multi-beam system, i.e., a system which emits a plurality of optical beams and scan them over a photosensitive drum along a plurality of lines at one time.
The multi-beam digital copying machine is provided with an optical system unit serving as an optical beam scanning apparatus. This optical system unit is mainly made up of: a plurality of semiconductor laser oscillators (hereinafter referred to as laser oscillators) which emits a plurality of optical beams; a rotatable multifaceted mirror, such as a polygon mirror, for reflecting the optical beams emitted by the laser oscillators toward a photosensitive drum and scanning them over the photosensitive drum; and optical elements, such as a collimator lens and an f-.theta. lens.
An image forming apparatus employing an optical beam scanning apparatus is proposed in a Jpn. Pat. Appln. KOKAI Publication No. 10-76704 (the contents of which are identical to those of Japanese Patent Application No. 8-233198). According to this publication, an optical beam position sensor produces a sensor output, and this sensor output is used for the positional control of optical beams.
In this type of image forming apparatus, the control target value of the power of the optical beam emitted from each laser oscillator (i.e., output laser power) is adjusted in such a manner that the power of an optical beam incident on both the photosensitive drum and optical beam position sensor takes a predetermined value (e.g., 300 .mu.W) at the time of shipment. Owing to this, an optical sensing signal is output from the optical beam position sensor, and a desirable electrostatic latent image can be formed on the photosensitive drum.
Although the photosensitive drum may vary in characteristic with time, an adequate electrostatic latent image must be formed at all times. To enable this, the control target value of the power of the optical beam emitted from each laser oscillator can be increased by a serviceperson. If this is done, however, it is likely that the power of the optical beam incident on both the photosensitive drum and the optical beam position sensor will exceed the predetermined power (e.g., 300 .mu.W) (the power will be 400 .mu.W, for example). Since this has adverse effects on the sensing output signal from the optical beam sensor, it will not be possible to perform accurate control (e.g., accurate positional control).
To put it differently, the amount of light sensed by the optical beam position sensor may vary, depending upon the characteristic change which the photosensitive drum may undergo with time. In some cases, the optical beam position sensor produces an error sensing output, which would result in a failure of control, e.g., accurate positional control.